XML 69 R58.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Millions | May 18, 2018 | May 11, 2018
--- | --- | ---
NSM Insurance HoldCo, LLC |  |
Goodwill [Line Items] |  |
Goodwill and intangible assets acquired |  | $ 383.0
Fresh Insurance Services Group Limited |  |
Goodwill [Line Items] |  |
Goodwill and intangible assets acquired | $ 54.6 |

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_GoodwillLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Goodwill and Intangible Assets + References No definition available. + Details
Name: | wtm_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedGoodwillandIntangibleAssets
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=wtm_NSMInsuranceHoldCoLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=wtm_FreshInsuranceServicesGroupLimitedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |